Relief circuits for semiconductor switches are known, for example, from German patent DE-OS No. 32 44 623 and are described in "Elektrotechnische Zeitschrift (etz)", Vol. 104, No. 7/8, pages 328 to 322", noting page 131, 1983. This known wiring network is shown in FIG. 1 and contains two switching-on relief choke coils L1, L2, a switching-off relief capacitor C.sub.A, a storage capacitor C.sub.SP, two switching-off relief diodes D10, D20 and a discharging resistor R.
The phase inverter shown in FIG. 1 contains semiconductor switches T1 and T2 wired antiparallel with bypass diodes D1 and D2, respectively. An upper and lower end B and C of the phase inverter is coupled to the positive and negative potential, respectively, of the feeding DC voltage source u.sub.D via first and second switching-on relief choke coils L1 and L2, respectively The relief choke coils L1, L2 act as current rise limiters.
The switching-off relief capacitor C.sub.A is coupled at one end to the load output A of the phase inverter Further, the series circuit including storage capacitor C.sub.SP and the first switching-off relief diode D10 is coupled between the upper end B of the phase inverter and the other end of the switching-off relief capacitor C.sub.A. The second switching-off relief diode D20 is coupled to the junction point of the series circuit and the other end of the switching-off relief capacitor C.sub.A and to the lower end C of the phase inverter. A discharge resistor R serves to break up the trapped wire energy stored in the switching-off relief choke coils and the storage capacitor during the switching-off process. The discharge resistor R couples the junction point between the storage capacitor C.sub.SP and the first switching-off relief diode D10 to the negative potential of the DC voltage source u.sub.D. In another embodiment (not shown), a feedback device called a "chopper circuit" is used instead of the discharge resistor R to feed the trapped wire energy back into the DC voltage source u.sub.D.
The prior wiring network has the disadvantage of having circuit-related asymmetry. Thus, the series circuit of the storage capacitor C.sub.SP and the switching-off relief capacitor C.sub.A has a relieving effect. On the other hand, when the lower semiconductor switch T2 is switched off, only the switching-off relief capacitor C.sub.A has the relieving effect. Such asymmetry causes the generally identical semiconductor switches T1, T2 to be protected differently. Therefore, when designing such a circuit, the different rates of rise occurring for the periodic positive blocking voltage at the respective switches when the semiconductor switches T1 and T2 are switched off must be taken into consideration. The prior circuit has the further disadvantage that pulse-like currents occur in the leads of the feeding source due to the discharge resistor R.
Additionally, if an energy recovery device is used instead of the discharge resister R, the trapped energy stored in the network is fed back into the source u.sub.D. This, however, corresponds to a "reactive commutation power" required by the inverter which must periodically be made available by the feeding DC voltage source u.sub.D for the purpose of switching off and on the switching relief network for the semiconductor inverter switches T1, T2. Furthermore, it is not possible and permissible for every application to subject the input DC voltage source u.sub.D to recharging energy pulsating in this manner.
Therefore, there is a need to provide a wiring network having a low-loss and symmetrical switching-off and on relief for the semiconductor switches of an inverter. Further, such a wiring network must operate without loading the feeding DC voltage source.